1. Field of the Invention
The invention relates to a motor control unit and an electric power steering system.
2. Discussion of Background
There is a conventional vehicle steering system such as an electric power steering system (EPS) that includes a motor control unit that is capable of controlling a brushless motor without detecting a rotation angle of the brushless motor. As a sensorless (resolver-less) drive control method in which a rotation angle sensor (motor resolver) is not used, the following method has been proposed. According to the method, an addition angle that corresponds to an amount of change in the motor rotation angle (hereinafter, referred to as “motor rotation angle change amount” where appropriate) in each computation period is computed, and current feedback control is executed on a rotating coordinate system that rotates in accordance with a motor rotation angle (control tingle) used in control, which is obtained by adding up the addition angles.
For example, a motor control unit described in US2010/0235051 executes torque feedback control based on a torque deviation (deviation between a target steering torque and an actual steering torque) that indicates the excess or deficiency of the actual torque relative to die target torque that should he generated by a motor, and computes a value obtained through the torque feedback control as an addition angle. Further, the motor control unit estimates a motor rotation angular velocity that corresponds to a motor rotation angle change amount based on an induced voltage. Under a predetermined condition, the value that is obtained through execution of the torque feedback control is corrected based on the estimated motor rotation angular velocity, and the corrected value is computed as the addition angle. In this way, it is possible to execute motor control with a higher accuracy.
The value of the addition angle is brought to a value close to a motor rotation angle change amount that should he achieved, through the correction based on the estimated motor rotation angle as described above. Because the torque deviation is reduced through the correction, a gain of the torque feedback control is preferably set to a low value so that generation of self-excited vibration due to excessive response is suppressed.
On the other hand, when the correction based on the estimated motor rotation angle is not executed, an addition angle is likely to be deviated from a motor rotation angle change amount that should be achieved Therefore, a torque deviation may be increased. Especially, in resolver-Jess control that is executed in accordance with a control angle that is an imaginary motor rotation angle used in the control, the deviation between the control angle and the actual rotation angle may be temporarily increased beyond a range of deviations at which a rotor of a motor is kept at a rotation position that corresponds to the control angle. Therefore, in order to achieve prompt recovery from such a state, a gain of the torque feedback control is required to be set to a high value to increase the response (followability).
However, in the related art described above, although the gain of the torque feedback control is variable based on the addition angle (absolute value) that is ultimately computed, whether the addition angle is corrected based on the estimated motor rotation angle is not taken into account. Therefore, the response is likely to be excessively high or low depending on whether the correction is made. As a result, the stability of the motor control may be reduced.